I gotta say, most of these translations miss the point. When I say "that's the least I can do" I mean "that's the least I can do"... I don't mean "не за что" and such. I guess in Russian you don't have a direct equivalent, but when I say "that's the least I can do" I'm actually sending a subtle message-- I want the other person to know that I'm willing to or should do more for him but I either can't because of certain constraints or I'm simply waiting for him to give me instructions of how I can help him more. That's what I mean by that.

That's why I like the version of "Да ладно! Это меньшее, что я могу делать"

Just one point of curiosity.... how do you pronounce меньшее? men-shee-ye?